Abstracted out the integer library used to support multiple backends (#373).
Every type now has a <B: Big> type parameter, where Big is a trait that abstracts over integer operations.
By default B is set to the DefaultBig backend, which is based on either num-bigint-dig (enabled by default) or rug (enabled with the rug feature).
Modified all use of integer serialization to use big-endian, to reduce room for error and match specification plans (see #76 in the spec).
